Scioly.org

mastersuperfan wrote:
I was intrigued by the 15-point math problem on page 11 and decided to try it myself. I'm getting an answer of 0.018 M, but the key says 0.00638 M. I think there may be an error in the first step of the key when [C6H5CO2-] is calculated, because I think it should be 0.290 M instead of 0.00333 M. Although I did the problem in a very different way, I am also getting the key's answer when I use 0.00333 M, so I don't think I'm making a mistake here. Could you check this? (On another note, I noticed that the key assigns point values to solving for [H+], but my solution doesn't require you to find it at all.)

Yep, checking right now. Yeah, it should be 0.290 M which changes the answer entirely. In terms of the problem--I wasn't around for most of the grading, so it might not have ended up so in reality, but if you got the right answer and the work looked reasonable you should have gotten all the points. I did get 0.106 M for the answer with the right concentration. Just wondering--what method did you use?

Well, this is embarrassing. I'll be fixing the key ASAP. Thanks for catching this. To my knowledge, most of the teams didn't do the problem so the point upset should be very minimal. Anyway, I'm glad someone finally did the problem...But I feel really bad for teams who should have gotten at least a few more points on this and didn't. Sorry guys. This isn't up to my personal standards as an event supervisor or a chemist, and definitely not up to the standards of quality MIT sets as a tournament.

As for when/where exams will get posted--ESs were allowed until 1/16 to submit changes so I'd expect a few more days before release.

mastersuperfan wrote:I think one focus next year should be on making sure all the grading is correct and double-checked, even if it takes some extra time. After looking through some of our teams' tests, I've noticed an egregious number of instances where entire sections or pages weren't added in calculating the final score—particularly in A&P, Expedes, Forensics, and Water Quality. It's kind of discouraging to know that your placing is substantially lowered below your actual performance due to forces beyond your control...

Just to clarify, we definitely checked scores in Forensics. Some of them may look like pages weren’t added because not every single graded test always had total page scores at the bottom, but rest assured we did make sure to add up all the points. While we were simultaneously grading essays and checking all the scores, we realized some of our volunteers went rogue on our grading rubrics without talking to us about what they were doing, so having that definitely was a huge setback in terms of time, so that also threw us another monkey wrench against trying to grade everything by awards time.

primitive_polonium wrote: I did get 0.106 M for the answer with the right concentration. Just wondering--what method did you use?

I think it would be a bit too long-winded to explain it in words so I've just attached an image of my work below (I didn't show the dilution calculations for the initial concentrations but the rest is there): https://i.imgur.com/IAm38L4.jpg

I'm still getting 0.0018 M for an answer, not 0.0106 M—not sure if there might be another error somewhere.

pikachu4919 wrote:Just to clarify, we definitely checked scores in Forensics. Some of them may look like pages weren’t added because not every single graded test always had total page scores at the bottom, but rest assured we did make sure to add up all the points. While we were simultaneously grading essays and checking all the scores, we realized some of our volunteers went rogue on our grading rubrics without talking to us about what they were doing, so having that definitely was a huge setback in terms of time, so that also threw us another monkey wrench against trying to grade everything by awards time.

I have added up all the points on each individual question at least five times on our test and it's always 36 points above what's written on the front. I have done the same with our other team's test and the individual point values match the total score when added up. Considering that our test had one page that earned exactly 36 points, I don't see any possibility besides that the page was simply missed. I don't mean to criticize the event—it was run very smoothly and efficiently overall—but when grading long tests gets rushed under a time constraint, mistakes like these slip through even with multiple people checking.

mastersuperfan wrote:

pikachu4919 wrote:Just to clarify, we definitely checked scores in Forensics. Some of them may look like pages weren’t added because not every single graded test always had total page scores at the bottom, but rest assured we did make sure to add up all the points. While we were simultaneously grading essays and checking all the scores, we realized some of our volunteers went rogue on our grading rubrics without talking to us about what they were doing, so having that definitely was a huge setback in terms of time, so that also threw us another monkey wrench against trying to grade everything by awards time.

I have added up all the points on each individual question at least five times on our test and it's always 36 points above what's written on the front. I have done the same with our other team's test and the individual point values match the total score when added up. Considering that our test had one page that earned exactly 36 points, I don't see any possibility besides that the page was simply missed. I don't mean to criticize the event—it was run very smoothly and efficiently overall—but when grading long tests gets rushed under a time constraint, mistakes like these slip through even with multiple people checking.

Email me a scan of your test and I'll check it out. They're in your packet.

mastersuperfan wrote: I'm still getting 0.0018 M for an answer, not 0.0106 M—not sure if there might be another error somewhere.

I decided to do the problem for fun, and I got .0017 M, which is close to .0018. I hope there aren't any math errors, since I let the calculator do the algebra! https://i.imgur.com/I6mpAt9.jpg

apc1234567 wrote:

mastersuperfan wrote: I'm still getting 0.0018 M for an answer, not 0.0106 M—not sure if there might be another error somewhere.

I decided to do the problem for fun, and I got .0017 M, which is close to .0018. I hope there aren't any math errors, since I let the calculator do the algebra! https://i.imgur.com/I6mpAt9.jpg

Not sure how you set it up, but 0.00172 M is just the molar solubility of PbF2 in water (check it with the Ksp).

The question only offers two significant figures, but to be more precise, my answer is around 0.0181 M, which is slightly higher than 0.00172 M. This makes sense because I don't think the buffer would drastically increase the solubility, not when F- is a weak base—the increase should be small.

mastersuperfan wrote:

apc1234567 wrote:

mastersuperfan wrote: I'm still getting 0.0018 M for an answer, not 0.0106 M—not sure if there might be another error somewhere.

I decided to do the problem for fun, and I got .0017 M, which is close to .0018. I hope there aren't any math errors, since I let the calculator do the algebra! https://i.imgur.com/I6mpAt9.jpg

Not sure how you set it up, but 0.00172 M is just the molar solubility of PbF2 in water (check it with the Ksp).

The question only offers two significant figures, but to be more precise, my answer is around 0.0181 M, which is slightly higher than 0.00172 M. This makes sense because I don't think the buffer would drastically increase the solubility, not when F- is a weak base—the increase should be small.

It turns out I put in the wrong value for 1/Ka (I flipped the sign of the exponent lmao) When I put in the right value, I got .0018M.

apc1234567 wrote:

mastersuperfan wrote:

apc1234567 wrote:
I decided to do the problem for fun, and I got .0017 M, which is close to .0018. I hope there aren't any math errors, since I let the calculator do the algebra! https://i.imgur.com/I6mpAt9.jpg

Not sure how you set it up, but 0.00172 M is just the molar solubility of PbF2 in water (check it with the Ksp).

The question only offers two significant figures, but to be more precise, my answer is around 0.0181 M, which is slightly higher than 0.00172 M. This makes sense because I don't think the buffer would drastically increase the solubility, not when F- is a weak base—the increase should be small.

It turns out I put in the wrong value for 1/Ka (I flipped the sign of the exponent lmao) When I put in the right value, I got .0018M.

Yep--this is the correct answer. I made an assumption that [H+] would remain constant but it doesn't. It held with the original typo but is not actually correct. I also got [0.0018].

Will put a note about this in the key; thanks guys!

Anyone know any way to contact the event supervisor for code buster? I have a few questions (specifically about the RSA)

Name wrote:Anyone know any way to contact the event supervisor for code buster? I have a few questions (specifically about the RSA)

The event supervisor emails should have been sent out to coaches last week.